1. Field of the Invention
The present invention relates to a wavelength-tuning and clock-synchronous system used in a wavelength division multiplexing (WDM) system or network, a WDM optical communication system in which wavelength and clock of a signal transmitted from a transmitter office are respectively tuned and synchronized based on a reference signal transmitted from a reference station, a WDM optical communication system in which a receiver station time-division demultiplexes wavelength division multiplexed signals of plural wavelengths based on a reference signal transmitted from a reference station, a WDM optical communication system in which these techniques are appropriately combined and the like.
2. Related Background Art
In wavelength division multiplexing (WDM) optical communication systems, different wavelengths are individually assigned to a plurality of transmitter offices or stations, and it is necessary to tune these wavelengths to oscillation wavelengths of the reference light source respectively, and at the same time to synchronize clocks of the transmitter stations, which determine a transmission speed of the system, with each other.
A prior art wavelength-tuning and clock-synchronous system in such a WDM optical communication system will be described with reference to FIG. 1. In FIG. 1, reference numeral 150 designates a reference station, and the reference station 150 has a temperature stabilization circuit 121 for stabilizing the oscillation wavelength of a laser diode (LD) 122. The laser diode 122 is a multimode-laser diode 122 for supplying light of reference or absolute wavelengths, an isolator for interrupting light from a transmission line or optical fiber 152 and a circuit 124 for generating a reference clock. Reference numerals 161 and 162 respectively designate first and second transmitter stations, and each of the transmitter stations 161 and 162 includes an O/E converter 125 for converting an input signal light to an electric signal, an auto-frequency control (AFC) circuit 126 for stabilizing the wavelength of an output light of an E/O converter 128 based on the electric signal from the O/E converter 125, an isolator 127 for interrupting light from the transmission line 152, an E/O converter for converting an electric signal from a transmission circuit 129 to an optical signal while the wavelength of the optical signal is being controlled by the AFC circuit 126, a transmission circuit 129 for processing a signal from a terminal which is synchronized with the clock from a transmission line or coaxial cable 154 to generate a digital signal, and a branching-combining (b-c) device 130 for branching and/or combining the optical signal. Reference numerals 131 and 132 respectively designate branching-combining devices disposed on the optical fiber 152. As described above, the transmission line 152 is, for example, an optical fiber, and the transmission line 154 is, for example, a coaxial cable.
Initially, the wavelength-tuning will be described. The reference station 150 supplies light of absolute wavelengths to the respective transmitter stations 161, 162, . . . , and the respective transmitter stations conduct tuning to assigned wavelengths. Thus, the wavelength tuning is achieved in each transmitter station. The multimode laser diode 122 is used as a reference light source for emitting light of absolute wavelengths, and each one of the wavelengths in its longitudinal mode is assigned to each transmitter office. Since the oscillation wavelength of the multimode laser diode 122 is changeable depending on a change in temperature, the oscillation wavelength needs to be stabilized by the temperature stabilization circuit 121. Thus, light of reference wavelengths is supplied to each transmitter station. The light of reference wavelengths output by the multimode laser diode 122 is transmitted to the optical fiber transmission line 152 via the isolator 123, and is branched by the respective b-c devices 131, 132, . . . to be input into the respective transmitter stations 161, 162, . . . Light of reference wavelengths input into the transmitter station is further branched by the b-c device 130 to be input into the O/E converter 125. At the same time, the output light of the E/O converter 128 is supplied through the isolator 127 and is branched by the b-c device 130. One of the divided lights is transmitted to the light transmission line 152, while the other one is input into the O/E converter 125.
At this time, the wavelength of the output light from the E/O converter 128 is previously set to a value that is in the vicinity of the wavelength assigned to the associated terminal. Further, the reference light incident on the O/E converter 125 and the signal light from the E/O converter 128 are converted to electric signals by the O/E converter 125. Therefore, a beat voltage, which corresponds to a wavelength difference between the reference light and the signal light from the E/O converter 128, is generated. The AFC circuit 126 controls the E/O converter 128 so that the amplitude of the beat signal is maintained at a constant value. That control signal is supplied to the E/O converter 128 as a control current, through a wavelength control terminal of the E/O converter 128. In the above manner, each transmitter station performs wavelength tuning based on light of reference wavelengths supplied from the reference station 150.
Turning to the clock synchronization, a stable clock generating circuit 124 is disposed in the reference station 150, and a reference clock is supplied to the respective transmitter stations 161, 162, . . . through the coaxial cable 154. The clock input into the transmitter station is supplied to the transmission circuit 129, and a signal from the terminal is processed thereby to generate the digital signal in the transmission circuit 129. The digital signal is then converted to the optical signal by the E/O converter 128 to be transmitted to the transmission line 152. Thus, all the transmitter stations 161, 162, . . . are synchronized with the reference station 150.
Another prior art WDM optical communication system will be described with reference to FIG. 2. In FIG. 2, reference numeral 250 designates a reference station, and the reference station 250 includes a temperature stabilization circuit 221 for stabilizing oscillation wavelengths of a laser diode 222. The laser diode 222 is a multimode laser diode 222 for emitting light of reference wavelengths. The reference station also includes an isolator for interrupting light from a transmission line 252. Reference numeral 240 designates a receiver station, and the receiver station 240 includes a demultiplexer 224 for demultiplexing the optical signals of wavelengths, an O/E converter 225 for converting the optical signal at a predetermined wavelength from the demultiplexer 224 to an electric signal, a timing extraction circuit 226 for extracting a timing component from a received signal to reproduce the clock, and a code conversion circuit 227 for converting a coded signal from the transmitter station to an original. A reference numeral 228 designates a branching-combining device for branching and/or combining the optical signal.
In the system of FIG. 2, wavelengths .lambda.A-.lambda.D are respectively assigned to the transmitter stations 261-264, multiplexed lights of such wavelengths are transmitted through the light transmission line 252, and the receiver station 240 demultiplexes the multiplexed wavelengths into signals of respective wavelengths. The reference station 250 supplies lights of stable absolute wavelengths to the respective transmitter stations, and each transmitter station stabilizes its own oscillation wavelength based on the reference wavelengths of reference light, as described in FIG. 1. The multimode laser diode 222 is used as a reference light source for emitting reference light of absolute wavelengths, and each one wavelength thereof in its longitudinal mode is assigned to each transmitter station. Since the oscillation wavelength of the multimode laser diode 222 varies depending on a change in temperature, the laser diode 222 supplies light of reference wavelengths under a condition under which the wavelengths are stabilized by the temperature stabilization circuit 221. The reference light output from the laser diode 222 passes through the isolator 223, is transmitted to the light transmission line 252, is branched by the b-c device 228 and enters the respective transmitter stations 261-264. In the respective transmitter stations, their light sources are respectively tuned to the assigned wavelengths of the reference light. Thereafter, each transmitter station performs an optical communication at the assigned wavelength. Thus, optical signals of wavelengths .lambda.A-.lambda.D are respectively supplied from the transmitter stations 261, 262, 263 and 264, are combined by the b-c device 228, and are transmitted to the receiver station 240 through the transmission line 252. In this example, a reference clock is not used, unlike the example of FIG. 1.
The optical signal input into the receiver station 240 is demultiplexed into signals of respective wavelengths .lambda.A-.lambda.D by the demultiplexer 224, and the signal of wavelength .lambda.A, for example, is converted to an electric signal by the O/E converter 225. The timing extraction circuit 226 extracts the timing component from the received signal to regenerate the clock. The timing extraction circuit 226 operates the code conversion circuit 227 at such a timing. The code conversion circuit 227 converts the coded signal transmitted from the transmitter station to its original signal and supplies the original signal to the terminal. Similarly, the signals at wavelengths .lambda.B-.lambda.D are respectively converted to electric signals at circuits 271, 272 and 273, and thereafter the electric signals are code-converted with the extracted clock. Thus, the coded signals at wavelengths .lambda.A-.lambda.D are respectively received. In the above-discussed manner, WDM signal is demultiplexed into signals at respective wavelengths by the receiver station 240 to achieve optical communication.
In the prior art example of FIG. 1, however, the light of reference wavelengths for wavelength-tuning and the reference clock for clock-synchronization are respectively transmitted via separate transmission lines from the reference station to the transmitter stations, in WDM communications. As a result, cost for transmission lines increases.
Further, in the prior art example shown in FIG. 2, optical receivers need to be used for respective wavelengths of the WDM communication, the transmitted code needs to be converted to such a code that has a timing component (e.g., CMI and RZ code) in the transmitter station so that the clock can be regenerated in the receiver station, and thus the size of circuits in transmitter and receiver stations increases.